In the field of semiconductor devices, formation of fine patterns has been rapidly developed. The size of wiring in a device was substantially 1 μm about 10 years ago, it is substantially 0.18 μm at the present time, and furthermore, devices having a wiring size of 0.13 μm have almost come into practical use. Further, researches and developments to manufacture semiconductor devices having a wiring size from 0.10 μm to 0.07 μm, or even of 0.05 μm have been started.
Furthermore, along with a higher speed of the semiconductor device, introduction of new materials is vigorously in study. For instance, as insulator, low dielectric constant (low-k) materials attract increasing attention, and the possibility that porous materials of organic materials and organic/inorganic composite materials are used for low-k materials and how to reduce the dielectric constant are recently actively studied. The development of such semiconductor devices causes various problems that have not been problematic so far.
For instance, in a cleaning process that is an important process in the manufacturing process of the semiconductors, so far, as a cleaning method of semiconductor wafers, a wet cleaning method in which a semiconductor is cleansed with a solution in which necessary additives are added to ultra pure water has been adopted. After the cleaning, it is general to rinse the wafer with ultra-pure water, followed by applying spin dryer in which the wafer is rotated to spin off water. As the additives, amine-based compounds or fluorinated compounds have been selected in accordance with applications.
However, with the miniaturization of semiconductor devices and the application of new materials, there have been caused problems by using the water-based cleaning method. One of them is in that since the water-based cleaning agent cannot permeate into fine via holes of a diameter of substantially 0.1 μm, the cleaning cannot be thoroughly performed. Though difference in the extent of permeation depends on the diameter of via hole and material, it is considered that the more the via hole is miniaturized, the more difficult the cleaning becomes owing to physical properties such as the interfacial tension and viscosity that liquid intrinsically has.
Furthermore, in the case of the porous new materials that have a lot of holes further finer than via hole, there have been the other problem that it is difficult to remove the cleaning liquid inside of the fine hole even if the cleaning liquid has permiated into the fine hole.
When water remains on a surface of a semiconductor wafer, various kinds of inconveniences are caused in later processes; accordingly, drying after the cleaning is important. In this connection, along with the miniaturization of the devices, occurrence of traces called watermark that remains after water is dried is also problematic. Furthermore, consuming a lot of precious water resource in the cleaning process cannot be said appropriate from a viewpoint of protection of the environment.
Similar problems have occurred also in the development process of the semiconductor wafers. In the development process of the semiconductor wafers, exposed resist material is developed with an aqueous solution of TMAH (tetramethyl ammonium hydroxide). The development process is followed by rinsing with ultra pure water further followed by drying by means of spin-dry. Accordingly, there are similar problems as in the cleaning process of the wafers. Furthermore, there is a problem that the projected portions of the pattern are destroyed by the capillary force generated in a gas-liquid interface and so on because a fine resist pattern is not such tough.
In order to solve the problems, recently, supercritical fluid has been studied to use for cleaning and as a rinse liquid. A supercritical state means a state above the critical temperature and critical pressure intrinsic to a substance; it is a fourth state of a substance that is neither solid, nor liquid, nor gas; in the supercritical liquid, particularly, intermediate properties of the liquid and the gas strongly appear. For instance, the density of the supercritical fluid is close to that of the liquid; however the viscosity and diffusion coefficient are close to that of the gas. Accordingly, the supercritical fluid has the density close to the liquid on one hand, and has movability and penetrating ability close to that of the gas on the other hand.
Industrially, carbon dioxide is most frequently used as the supercritical fluid; this is because its critical pressure is low such as 7.3 MPa, its critical temperature is close to room temperature such as 31 degree centigrade, and it is nonflammable, inexpensive and harmless. Supercritical carbon dioxide has many excellent properties as a fluid that can be used in place of water in the cleaning process of the semiconductor devices.
First, the supercritical carbon dioxide can easily penetrate into the via holes and fine pores of the porous material and can be easily removed therefrom. Accordingly, the difficulty in the cleaning accompanying the miniaturization of the device can be solved. In the next place, the supercritical carbon dioxide, having the density close to that of the liquid as mentioned above, can contain a large quantity of a additives and a co-solvent; in other words, it means that the supercritical carbon dioxide has cleaning ability comparable to ordinary liquids. Still furthermore, since there is no need of using water in the cleaning process, all of the aforementioned problems such as the problem of remaining water, the problem of the watermark in the cleaning process, the problem of pattern destruction due to the interfacial tension and the problem of environmental destruction can be overcome by use of the supercritical carbon dioxide.
In this connection, the present invention intends to provide the most preferable method when substances to be processed such as semiconductor wafers are processed with supercritical carbon dioxide.